Such a servo steering unit has been disclosed in the older German patent application P 41 26 020.1. In said servo steering unit, the spring-loaded closing member of the inlet seat valve can be released by means of a stop disc. This prevents a stronger actuating force from being needed for opening the inlet seat valve than for opening the outlet seat valve.
To obtain the effect described, a very thorough coordination of two actuation spaces is required. A first space between an actuating tappet and the stop disc must here be smaller than a second space between the closing surface of a closing member of the outlet seat valve and a valve seat of the outlet seat valve situated on the closing member of the inlet seat valve. Since the actuation paths of the seat valves are in the range of a few tenths of a millimeter, the coordination is very difficult. In addition, in said servo steering unit only one centering spring is active at the beginning of the adjustment of the steering valve away from the neutral position. The pre-load force of the pressure spring acting upon the closing member of the inlet seat valve will increase only after overcoming the first space and the actuating tappet abuts on the stop disc. This result is contrary to the requirement generally existing in regard to steering valves of an increased stiffness of the steering in the central position.